The present invention relates to waveform synthesis and, more particularly, to a synthesis of waveforms consisting of a series of pulse-width modulated (PWM) pulses. A major objective of the present invention is to provide for more cost-effective control of alternative-current (inductive) motors.
Practical electric vehicles (EVs) are strongly desired as an alternative to vehicles powered by fossil fuels to reduce pollution and dependence on energy sources. Direct-current (DC) motors have the advantage of straight forward speed control, but have a relatively low power/mass ratio. Alternating-current (AC) motors have a better power/mass ratio, with 1000 horsepower motors being readily available. Currently, the most promising approach to powering an EV appears to be a high-speed AC inductive motor and a 3-phase IGBT (insulated-gate bipolar transistor) inverter. However, precise speed control of such a motor is a challenge.
The speed of an AC motor is governed largely by the frequency of the applied alternating current. Speed control of AC motors thus depends on having an AC waveform with precisely controllable frequency. A practical solution for vehicles is to approximate the time-varying energy distribution of a sinusoidal AC waveform using one or more power pulse trains. The pulses are of constant amplitude, but vary in width as a function of the phase of the sinusoidal wave to be approximated. Although the pulses scarcely resemble sinusoidal waves, the current that flows in the motor windings is nearly sinusoidal because the inductance and back electromotive force of the motor windings filter out the high-frequency components of the power pulse train.
The required power pulse train can be generated by amplifying a drive pulse train that has the desired timing and pulse widths. Suitable DC powered power-pulse generation circuits are commercially available. A drive pulse train can be readily generated at logic power levels (e.g., +5 Volts) by reading out pulse-width values sequentially from a previously generated look-up table. In theory, such a table could be read out at different rates to provide for different frequencies. However, the power delivered would fall linearly with frequency, whereas vehicles tend to require more power at lower speeds. Furthermore, hardware with sufficiently precise variability, such as timers, is a cost concern.
An alternative approach would be to provide different tables for different frequencies. However, the storage requirements for a suitable number of tables would be burdensome. One widespread approach generates needed tables on the fly as follows. A digitized sine wave is read out at a selected rate; a digital-to-analog conversion provides an analog sine wave. This sine wave is then sampled at a constant frequency to provide a pulse-width look-up table. When this table is read out, a PWM pulse train is provided with the desired frequency. The problems with this approach include the costs of the timing and digital-to-analog sampling. In addition, there is an inevitable loss of precision in the conversions into and out of the analog domain.
All-digital PWM generation is desired to avoid analog processing. One approach uses an interpolator to modify pulse-width modulation values for a table corresponding to a given frequency to obtain values for a desired frequency. A disadvantage of this approach is the computational burden imposed by the interpolations, which also involve some loss of precision. This computation burden can adversely affect precision, responsiveness, and cost of an incorporating system. What is needed is an all-digital PWM generator that requires substantially less computational power than other PWM generators.